Research And Application On Protection Technology Of Shallow Water Subsea Span Pipeline

After 20 years’ development, Shengli offshore oilfield has attained crude oil production scale of 3.5 million tons per year. Compared with other sea areas, because of the serious overall scouring seabed and the severe geological condition, the phenomenon of existed subsea pipeline span caused by scouring often arises and it makes serious trouble to pipeline operation safety. Presently, research on span subsea pipeline protection at home and abroad mostly focus on single seabed soil parameters and single protection measures, and the research of comprehensive protection technology of span subsea pipeline under the condition of overall scour seabed and silt/soft clays is less and lack.Combining the project instance, flow visualization is used to simulate the flow around span pipeline in this paper. Vortices formation around span pipeline and influencing factors of span pipeline caused by scouring have been clear, and these research results have provided an theoretic basis of protection for subsea span pipelines.The seabed scouring model has been established. Overall scouring seabed is simulated as flat deep chamfer model in the current direction and varying deep chamfer in the pipeline axial direction. Local scouring seabed is simulated as cosine distribution model in the current direction. The influence of span height, current velocity, boundary conditions, span length to deflect behave of various pipelines has been clear. the deflection and bending stress increase and deflection grows faster than stress as the span length increasing, and then bending stiffness fails. If span length continues to increase, the pipe finally will come into contact with the seabed and maybe return to safty state. This paper points out that attention should be paid to prioritize local scour of subsea pipeline, optimization of concrete weight coating thickness and optimizing pipe in pipe structure.Based on the project instance, vortices formation around span pipelines is analyzed. Dynamic stress of in-line and cross-flow Vortex Induced Vibrations(VIV) under different span height, current velocity, span length is studied and the fatigue life of the pipeline is predicted.According to Shengli offshore oilfield environmental data and geological conditions, mathematical modeling and experiments are performed to investigate a new method of protecting span pipelines using concrete mattress with geo-textile. The practical application shows that this method has advantages such as integrality, good adaptability to unstable seabed, suited to mechanized installation and so on.It can provide effective protection for span and bare subsea pipelines, and it’s also an economical and effective method for overall erosion seabed area.